Symmetric cancelling anti-striation circuit

ABSTRACT

A striation correction circuit  300  is arranged to apply a first striation correction current to the first fluorescent lamp L 1  and a second striation correction current to the second fluorescent lamp L 2 . A first voltage appearing across the first fluorescent tube L 1  due to the first striation correction current is substantially similar in magnitude but has inverted polarity with respect to a second voltage across the second fluorescent tube L 2  due to the second striation correction current. Detection of an end-of-life condition of a fluorescent lamp is facilitated.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application Ser.No. 60/429,706 filed Nov. 27, 2002, and U.S. provisional applicationSer. No. 60/474,829 filed May 30, 2003 which is incorporated herein byreference.

The invention relates to fluorescent lamp driving circuits. Moreparticularly the invention relates to improved techniques forconfiguring a two-lamp fluorescent lamp for end-of-life detection withan anti-striation circuit.

In the field of fluorescent lamps it is known that some lamps exhibitstriations when dimmed to very low levels. One way to remove thestriations is to inject a small DC component into a lamp drivingcurrent. Some lamps with a small diameter require ballasts having anend-of-life (EOL) detection circuit. A fluorescent lamp tube nearingfailure will typically exhibit a fluctuation in impedance that willappear to a detection circuit. As a lamp begins to fail, the EOL circuitsenses a change in the DC voltage at the lamp. However, the impedance ofa lamp operated at low light output levels is much higher than the lampimpedance of full brightness operation. Therefore, even a small DCcurrent multiplied by the lamp impedance may result in a significantvoltage. The EOL circuit cannot distinguish between a lamp voltage dueto low-light lamp operation and a voltage change due to an impendinglamp failure. An erroneous shut down of the ballast may then betriggered by the EOL sensing circuit. The EOL detection is even lessreliable with series connected two lamp configurations, particularlywhen the lamps have a higher voltage at low dim levels since the lampvoltages will add.

FIG. 1 shows a circuit diagram for an anti-striation circuit for aseries-connected two-lamp configuration having end-of-life detection. Anisolation transformer T1 couples power to a series connected two-lampload L1 and L2. A resistor R1 and a diode D1 across the load provide ananti-striation circuit for dimmed operation. A small DC current (Idc),is injected into the load by the voltage drop across R1. A capacitor C1senses a change in the potential voltages VL1 and VL2 across the lamps.The scalar impedance of each lamp RL1, RL2, for lamps L1 and L2respectively is:RL=VL/Idc, and   (1)the net DC voltage on the capacitor is given by the relationship:VL1+VL2=2*Idc*RL.  (2)

Generally, the EOL shutdown voltage level must be set higher than theexpected DC voltage level resulting from dimmed lamp operation suchthat:EOL shutdown DC level>2*Idc*RL.  (3)

It would therefore be desirable to provide an improved anti-striationcircuit configuration that addressed these and other limitations.

The present invention is directed to a fluorescent lamp circuit. Inaccordance with the invention, a power source is selectively arranged todeliver power to a load, a first fluorescent lamp is coupled to thepower source and a second fluorescent lamp coupled in series to thefirst fluorescent lamp and coupled to the power source. A striationcorrection circuit is coupled to the power source and coupled to thefirst and second fluorescent lamps. The striation correction circuit isarranged to apply a first striation correction current to the firstfluorescent lamp and a second striation correction current to the secondfluorescent lamp. A first voltage appearing across the first fluorescenttube due to the first striation correction current is substantiallysimilar in magnitude but has inverted polarity with respect to a secondvoltage across the second fluorescent tube due to the second striationcorrection current.

In accordance with another aspect of the invention, a method of reducingstriations in a fluorescent lighting circuit is provided. A firststriation correction current and a second striation correction currentare generated. The first striation correction current is applied to afirst fluorescent lamp. The second striation correction current isapplied to a second fluorescent lamp. The first fluorescent lamp and thesecond fluorescent lamp are coupled in series. A first voltage appearingacross the first fluorescent lamp due to the first striation correctioncurrent is substantially similar in magnitude but has inverted polaritywith respect to a second voltage appearing across the second fluorescentlamp due to the second striation correction current.

The foregoing and other features and advantages of the invention areapparent from the following detailed description of exemplaryembodiments, read in conjunction with the accompanying drawings. Thedetailed description and drawings are merely illustrative of theinvention rather than limiting, the scope of the invention being definedby the appended claims and equivalents thereof.

FIG. 1 illustrates a prior art anti-striation circuit for a fluorescentlamp ballast having end-of-life detection.

FIG. 2 illustrates a fluorescent lamp circuit having a striationcorrection circuit and end-of-life detection in accordance with theinvention.

FIG. 3 illustrates a second embodiment of the fluorescent lamp circuitof FIG. 2 in accordance with the present invention.

FIG. 4 illustrates another embodiment of a fluorescent lamp circuit asin FIGS. 2 and 3.

FIG. 5 is a flow diagram of a method for reducing striations in afluorescent lamp system.

FIG. 6 illustrates another embodiment of a fluorescent lamp circuit asin FIG. 2 in accordance with the present invention.

In the following description the term “coupled” means either a directconnection between the things that are connected, or a connectionthrough one or more active or passive devices that may or may not beshown, as clarity dictates.

FIG. 2 illustrates a fluorescent lamp circuit having a striationcorrection circuit and end-of-life detection in accordance with theinvention. FIG. 2 shows an isolation transformer T1 coupled to aclosed-loop circuit comprising: a capacitor C1; a striation correctioncircuit further comprising a first lamp correction circuit having adiode D1 in series with a resistor R1, and a second lamp correctioncircuit having a diode D2 in series with a resistor R2; and, first andsecond series-coupled fluorescent lamps, L1 and L2. The first and secondlamp correction circuits are coupled in series with the diodes, D1 andD2, opposing one another in current sense. The first fluorescent lamp L1is shown in parallel with the first lamp correction circuit, D1 and R1.The second fluorescent lamp L2 is shown in parallel with the second lampcorrection circuit, D2 and R2. The capacitor C1 is shown in series withthe transformer T1 and the lamp circuits.

Power is selectively coupled to the fluorescent lamp circuit through thetransformer T1. A dimmable switch-mode or PWM-type ballast is generallycoupled to the transformer T1 primary (not shown). The transformer T1 istypically an isolation transformer and may have one or more taps. Thediodes, D1 and D2, are any suitable diodes having a power ratingcommensurate with the required DC striation correction currents, andtypically will have the same rated values within the standardmanufacturer tolerance ranges. In one embodiment (not shown), D1 and D2are transistors configured as diodes. The resistors, R1 and R2, are anysuitable resistors for providing a voltage drop to generate a DCcurrent, and generally have the same rated value within standardmanufacturing tolerance ranges. The resistors, R1 and R2, are generallymetal film types, but carbon and wire wound resistors areinterchangeable. In one embodiment (not shown), R1 and R2 aretransistors configured as resistive loads. The fluorescent lamps, L1 andL2, are any suitable fluorescent lamps, typically of narrow diameter anddimmable. The capacitor C1 is any capacitor suitable for sensing a loopDC voltage change (EOL) due to an end-of-life condition on a fluorescenttube. In one embodiment, an EOL detection circuit is coupled to thecapacitor C1 (not shown.)

In operation, a DC current Idc is induced in each lamp correctioncircuit by applying power to the transformer T1 primary. A voltageacross the secondary of the transformer T1 causes a voltage drop acrossresistors R1 and R2. Symmetric DC currents are injected to thefluorescent lamps L1 and L2, due to the orientation of the diodes D1 andD2. As shown in FIG. 2, the current Idc injected to the fluorescentlamps L1 and L2 is substantially equivalent in magnitude within atolerance range, but opposite in sense. The opposing sense of theinjected current Idc causes the resulting DC voltages on the fluorescentlamps, L1 and L2, to be opposite in polarity and therefore substantiallyeliminated from the loop voltage. Therefore, under normal operation ofthe fluorescent lamp circuit, the net DC voltage on the capacitor C1 iszero. The shutdown threshold voltage for the EOL sensing circuit may beconfigured based upon the fluorescent lamp characteristics withoutregard to the striation correction circuits.

FIG. 3 illustrates a second embodiment of the fluorescent lamp circuitof FIG. 2 in accordance with the present invention. FIG. 3 shows afluorescent lamp circuit 300 comprising a transformer T4 coupling aballast circuit 310 to a closed-loop circuit comprising a capacitor C1;a striation correction circuit comprising a first lamp correctioncircuit having a diode D1 in series with a resistor R1 and a second lampcorrection circuit having a diode D2 in series with a resistor R2; firstand second series-coupled fluorescent lamps, L1 and L2; and, an EOLdetection circuit comprising current sense transformer T5, capacitorsC9, C10, and C11 and C12, and, transformers T9, T10, T11. The first andsecond lamp correction circuits are coupled in series with the diodes,D1 and D2, opposing one another in current sense. The first fluorescentlamp L1 is shown in parallel with the first lamp correction circuit, D1and R1. The second fluorescent lamp L2 is shown in parallel with thesecond lamp correction circuit, D2 and R2. The capacitor C1 is shown inseries with the transformer T1 and the lamp correction circuits.Capacitor C12 is shown coupled in series to the current sensetransformer T5. Capacitor C9 and transformer T9 are shown in series withthe current sense transformer T5 and a first end of the firstfluorescent lamp L1. Capacitor C10 and transformer T10 are shown inseries with the lamp correction circuits and a second end of the firstfluorescent lamp L1 and a first end of the second fluorescent lamp L2.Capacitor C11 and transformer T11 are shown in series with the currentsense transformer T5 and a second end of the second fluorescent lamp L2.In one embodiment (not shown), additional components such as currentsources, pass transistors and bias resistors are included in fluorescentlamp circuit 300. EOL detection circuits will be known to those skilledin the art and will not be further discussed.

In operation, the fluorescent lamp circuit 300 provides fluorescent lampstriation correction for dimmed operation and reliable end-of-lifedetection. As in FIG. 2, a DC current Idc is induced in each lampcorrection circuit by applying power to the transformer T4 primary. Avoltage across the secondary of the transformer T1 causes a voltage dropacross resistors R1 and R2. Symmetric DC currents are injected to thefluorescent lamps L1 and L2. The current Idc injected to the fluorescentlamps, L1 and L2, is substantially equivalent in magnitude within atolerance range, but opposite in sense. The opposing sense of theinjected current Idc causes the resulting DC voltages on the fluorescentlamps, L1 and L2, to be opposite in polarity and therefore substantiallyeliminated from the loop voltage. Therefore, under normal operation ofthe fluorescent lamp circuit 300, the net DC voltage on the capacitor C1is zero. The shutdown threshold voltage for the EOL sensing circuit maybe configured based upon the fluorescent lamp characteristics withoutregard to the striation correction circuits. In one embodiment (notshown), two or more fluorescent lamp circuits 300 may be configured forparallel operation with a combined power source.

FIG. 4 illustrates yet another embodiment of a fluorescent lamp circuitas in FIGS. 2 and 3. In particular, FIG. 4 shows a detailed schematicincluding an embodiment of a fluorescent lamp circuit as implemented inFIG. 3. A skilled practitioner will recognize components of a standardballast circuit depicted in FIG. 4. The design and operation of ballastcircuits is known to skilled practitioners and therefore the componentsand operation of the ballast circuit portions of FIG. 4 will not bediscussed. Of relevance to the present invention in FIG. 4 is astriation correction circuit comprising a first lamp correction circuithaving a diode D24 in series with a resistor R38 and a second lampcorrection circuit having a diode D23 in series with a resistor R38A; anEOL detection circuit comprising current sense transformer T5,capacitors C27, C28, and C29 and, transformers T1-C, T1-8, T1-9. Thefirst and second lamp circuits are coupled in series with the diodes,D24 and D23, opposing one another in current sense. The firstfluorescent lamp resides between terminals RED-A-RED-B and YEL-A-YEL-Bwhich is shown in parallel with the first lamp correction circuit, D24and R38. The second fluorescent lamp resides between terminalsYEL-A-YEL-B and BLU-A-BLU-B which is shown in parallel with the secondlamp correction circuit, D23 and R38A. The capacitor C25 is shown inseries with the transformer T4 and the lamp correction circuits.Capacitor C31 is shown coupled in series to the current sensetransformer T5. Capacitor C29 and transformer T1-9 are shown in serieswith the current sense transformer T5 and a first end of the firstfluorescent lamp L1. Capacitor C27 and transformer T1-C are shown inseries with the lamp correction circuits and a second end of the firstfluorescent lamp L1 and a first end of the second fluorescent lamp L2.Capacitor C26 and transformer T1-9 are shown in series with the currentsense transformer T5 and a second end of the second fluorescent lamp L2.Additional components such as current sources, pass transistors and biasresistors included in fluorescent lamp circuit will be understood by theskilled practitioner and will not be discussed.

In the following process description certain steps may be combined,performed simultaneously, or in a different order.

FIG. 5 is a flow diagram of a method for reducing striations in afluorescent lamp system. Process 500 begins in step 510. In step 510, afirst striation correction current and a second striation correctioncurrent are generated. The striation correction currents may be createdat any time power is applied to the fluorescent lamp system. Generally,the first and second striation correction currents are simultaneouslygenerated responsive to the application of a power source, as in thevoltage drop across resistors R1 and R2. The striation correctioncurrents are DC currents of a magnitude that reduces striations influorescent lamps operated at low light levels. In one embodiment,generation of the striation correction currents is selectably controlledbased on the operational status of the fluorescent lamp. For instance,detection of an end-of-life condition by an EOL circuit such asdescribed in FIGS. 3 and 4 may trigger a shut down of the striationcorrection current generation. In another embodiment, generation of thestriation correction currents is selectably controlled based on thelight output configuration of the lamp. For instance, selection ofnormal light level lamp operation may trigger a shutdown of striationcorrection current generation, whereas selection of low light level lampoperation may trigger striation correction current generation.

In step 520, the first striation correction current is applied to afirst fluorescent lamp L1. Application of the first striation correctioncurrent may occur at any time during or after generation of thecorrection current. Typically, application of the first striationcorrection current is concurrent with current generation.

In step 530, the second striation correction current is applied to asecond fluorescent lamp L2. Application of the second striationcorrection current may occur at any time during or after generation.Typically, application of the second striation correction current isconcurrent with both generation of the second striation current andgeneration and application of the first striation correction current.The second striation correction current is applied in an opposite senseto the application of the first striation correction current such that afirst voltage appearing across the first fluorescent lamp L1 resultingfrom the first striation correction current is substantially similar inmagnitude and having inverted polarity with respect to a second voltageacross the second fluorescent tube L2 resulting from the secondstriation correction current.

FIG. 6 illustrates another embodiment of a fluorescent lamp circuit asin FIG. 2 in accordance with the present invention. FIG. 6 shows onepair of fluorescent lamps L1, L2, with an additional pair of fluorescentlamps L11, L22 arranged in series with the pair of fluorescent lamps L1,L2. An additional striation correction circuit including resistors R11,R22 and diodes D11, D22 corresponds to the additional pair offluorescent lamps L11, L22. The additional pair of fluorescent lampsL11, L22 and additional striation correction circuit including resistorsR11 , R22 and diodes D11, D22 are all coupled to isolation transformerT1 and the power source, In this example, the lamp correction circuitincluding resistor R22 and diode 22 includes the diode D22 as atransistor.

While the preferred embodiments of the invention have been shown anddescribed, numerous variations and alternative embodiments will occur tothose skilled in the art. Accordingly, it is intended that the inventionbe limited only in terms of the appended claims.

1. A fluorescent lamp circuit, comprising: a power source selectivelyarranged to deliver power to a load; a first fluorescent lamp coupled tothe power source; a second fluorescent lamp coupled in series to thefirst fluorescent lamp and coupled to the power source; an end-of-lifedetection circuit coupled to the first and second fluorescent lamps; anda striation correction circuit coupled to the power source and coupledto the first and second fluorescent lamps that is arranged to apply afirst striation correction current to the first fluorescent lamp and asecond striation correction current to the second fluorescent lampwherein a first voltage appearing across the first fluorescent lampresulting from the first striation correction current is substantiallysimilar in magnitude and having inverted polarity with respect to asecond voltage across the second fluorescent lamp resulting from thesecond striation correction current.
 2. The circuit of claim 1 whereinthe end-of-life detection circuit comprises a capacitor arranged inseries with the first and second fluorescent lamps to sense voltagechanges in a closed-loop circuit with the power source and the first andsecond fluorescent lamps.
 3. The circuit of claim 2 wherein theend-of-life detection circuit further comprises a current sensetransformer.
 4. The circuit of claim 2 wherein net DC voltage on thecapacitor is zero.
 5. The circuit of claim 1 wherein the striationcorrection circuit comprises a first lamp correction circuit forgenerating the first striation correction current in the firstfluorescent lamp and a second lamp correction circuit for generating thesecond striation correction current in the second fluorescent lamp. 6.The circuit of claim 5 wherein the first lamp correction circuit isarranged in parallel with the first lamp and the second lamp correctioncircuit is arranged in parallel with the second lamp and wherein thefirst and second lamp circuits are in series.
 7. The circuit of claim 6wherein the first lamp correction circuit and the second lamp correctioncircuit each comprise a diode in series with a resistor and wherein thefirst and second lamp correction circuits are arranged symmetricallywith the diodes opposing one another other.
 8. The circuit of claim 5wherein the first lamp correction circuit and the second lamp correctioncircuit comprises at least one transistor.
 9. The circuit of claim 1wherein the power source is a fluorescent lamp ballast coupled to thefirst and second fluorescent lamps through an isolation transformer. 10.The circuit of claim 1 wherein the fluorescent lamp circuit comprises atleast one additional pair of fluorescent lamps and at least oneadditional corresponding striation correction circuit all coupled to thepower source and wherein the at least one additional pair of fluorescentlamps are arranged in series with the first and second fluorescentlamps.
 11. The circuit of claim 1 wherein the first and second striationcorrection currents are DC signals and wherein the first striationcurrent is opposite in sense to the second striation current.
 12. Thecircuit of claim 1 wherein the first striation correction current andthe second striation correction current are substantially equivalent inmagnitude.
 13. A method of reducing striations in a fluorescent lightingsystem, comprising: generating a first striation correction current anda second striation correction current; applying the first striationcorrection current to a first fluorescent lamp; applying the secondstriation correction current to a second fluorescent lamp wherein thefirst fluorescent lamp and the second fluorescent lamp are coupled inseries and wherein a first voltage appearing across the firstfluorescent lamp resulting from the first striation correction currentis substantially similar in magnitude and having inverted polarity withrespect to a second voltage appearing across the second fluorescent lampresulting from the second striation correction current; and sensing avoltage change in the fluorescent lighting circuit indicative of afluorescent tube end-of-life condition wherein an end-of life detectioncircuit is coupled to the first and second fluorescent lamps.
 14. Themethod of claim 13 wherein the end-of-life detection circuit comprises acapacitor arranged in series with the first and second fluorescentlamps.
 15. The method of claim 13 wherein a striation correction circuitcomprises a first lamp correction circuit for generating the firststriation correction current in the first fluorescent lamp and a secondlamp correction circuit for generating the second striation correctioncurrent in the second fluorescent lamp.
 16. The method of claim 15wherein the first lamp correction circuit is arranged in parallel withthe first lamp and the second lamp correction circuit is arranged inparallel with the second lamp and wherein the first and second lampcorrection circuits are in series.
 17. The method of claim 16 whereinthe first lamp correction circuit and the second lamp correction circuiteach comprise a diode in series with a resistor and wherein the firstand second lamp correction circuits are arranged symmetrically with thediodes opposing one another other.
 18. The method of claim 16 whereinthe first lamp correction circuit and the second lamp correction circuitare comprised of at least one component selected from the groupconsisting of a transistor, a resistor, a diode, a capacitor and aninductor.
 19. The method of claim 13 wherein the fluorescent lampcircuit comprises at least one additional pair of fluorescent lamps andat least one additional corresponding striation correction circuit allcoupled to the power source and wherein the at least one additional pairof fluorescent lamps are arranged in series with the first and secondfluorescent lamps.
 20. The method of claim 13 wherein the firststriation correction current and the second striation correction currentare substantially equivalent in magnitude.
 21. A system for reducingstriations in a multi-tube fluorescent lamp assembly, comprising: meansfor generating a first striation correction current and a secondstriation correction current; means for applying the first striationcorrection current to a first fluorescent lamp; means for applying thesecond striation correction current to a second fluorescent lamp; andmeans for sensing a voltage change in the fluorescent lighting circuitindicative of a fluorescent tube end-of-life condition wherein anend-of-life detection circuit is coupled to the first and secondfluorescent lamps; wherein the first fluorescent lamp and the secondfluorescent lamp are coupled in series and wherein a first voltageappearing across the first fluorescent lamp resulting from the firststriation correction current is substantially equal in magnitude andhaving inverted polarity with respect to a second voltage appearingacross the second fluorescent lamp resulting from the second striationcorrection current.
 22. The system of claim 21 wherein the firststriation correction current and the second striation correction currentare substantially equivalent in magnitude.